[docs] Fix CSS theme variables section (#43439)

docs/data/material/integrations/nextjs/nextjs.md

@@ -124,19 +124,16 @@ To learn more about theming, check out the [theming guide](/material-ui/customiz
 
 #### CSS theme variables
 
-If you want to use [CSS theme variables](/material-ui/customization/css-theme-variables/overview/), use the `extendTheme` and `CssVarsProvider` utilities instead:
+To use [CSS theme variables](/material-ui/customization/css-theme-variables/overview/), enable the `cssVariables` flag:
 
 ```diff title="src/theme.ts"
  'use client';
--import { createTheme } from '@mui/material/styles';
-+import { extendTheme } from '@mui/material/styles';
-
- // app/layout.tsx
--import { ThemeProvider } from '@mui/material/styles';
-+import { CssVarsProvider } from '@mui/material/styles';
+ const theme = createTheme({
++  cssVariables: true,
+ });
 ```
 
-Learn more about [the advantages of CSS theme variables](/material-ui/customization/css-theme-variables/overview/#advantages).
+Learn more about [the advantages of CSS theme variables](/material-ui/customization/css-theme-variables/overview/#advantages) and how to [prevent SSR flickering](/material-ui/customization/css-theme-variables/configuration/#preventing-ssr-flickering).
 
 ### Using other styling solutions
 
@@ -367,11 +364,13 @@ To learn more about theming, check out the [Theming guide](/material-ui/customiz
 
 #### CSS theme variables
 
-If you want to use [CSS theme variables](/material-ui/customization/css-theme-variables/overview/), use the `extendTheme` and `CssVarsProvider` instead:
+To use [CSS theme variables](/material-ui/customization/css-theme-variables/overview/), enable the `cssVariables` flag:
 
-```diff title="pages/_app.tsx"
--import { ThemeProvider, createTheme } from '@mui/material/styles';
-+import { CssVarsProvider, extendTheme } from '@mui/material/styles';
+```diff title="src/theme.ts"
+ 'use client';
+ const theme = createTheme({
++  cssVariables: true,
+ });
 ```
 
-Learn more about [the advantages of CSS theme variables](/material-ui/customization/css-theme-variables/overview/#advantages).
+Learn more about [the advantages of CSS theme variables](/material-ui/customization/css-theme-variables/overview/#advantages) and how to [prevent SSR flickering](/material-ui/customization/css-theme-variables/configuration/#preventing-ssr-flickering).